Probers are trying to find out whether any of New York’s Finest belonged to a notorious after-hours sex club, after photos of three men – with the letters NYPD on the back – turned up in a raid, sources said.

The photos were among 200 identification pictures confiscated when cops swooped down on Sights in Newark on March 17.

Internal Affairs investigators think the three patrons they’re looking into wrote “NYPD” on the backs of the pictures when they joined and were asked what they did for a living.

The photos were picked up yesterday by the NYPD from Newark vice detectives, who arrested 31 people, including two New Jersey state correction officers who were acting as bouncers.

A Newark police spokesman said numerous other membership photos had the names of various other New York and New Jersey law-enforcement agencies written on them, and they were trying to determine if they also were cops, court officers or troopers before releasing any more information.

When a patron showed up for the first time at the club he was required to pay a $25 membership fee and have his photo taken, sources said. No names were printed on the photos.

They were also asked about their jobs and many said they were in law enforcement. Officials think some of them lied, hoping for a break on the price – and an NYPD spokesman said last night he had “no reason to believe at this time” the three were actually city cops.

Patrons were also patted down for weapons and cell phones and pagers were confiscated at the door and returned later.

Once inside, customers could drink from a juice bar while watching naked dancers on a stage. The women then would come down off the stage and take a customer into one of four small side rooms, equipped with lounge chairs, and engage in sex for up to $100 for 10 minutes.

The show consisted of a patron and a woman engaging in live sexual action behind a large scrim on which their silhouettes would be projected by klieg lights.